The simple spores are produced within colourless tubercules called nuclei, variously situated upon the plant, as at fig. 23 _a_, _c_. These nuclei contain many microscopic spores. Sometimes the nuclei are enclosed in conceptacles or ovate sacs, which are either perforate or not at the apex. These contain many microscopic strings of cells like jointed threads, and the endochrome in each joint of these threads is converted into a spore successively from the summit downward. Sometimes the endochrome in one or two joints only, becomes a spore whether terminal or central, and when the spores break through the joint wall and fall off from the threads, they are collected without any definite order into a mass within the nuclei. Sometimes new joints or cells are produced on the threads when the old ones have yielded their fruit. Occasionally a globose nucleus contains several secondary nucleoli full of spores. In every instance, the perfect spore is a dense grumous mass surrounded by a hyaline sub-gelatinous coat consisting of at least two membranes. The situation, mode of growth, and structure of the nuclei vary almost infinitely, and together with the structure of the frond afford the distinctive marks by which the genera are separated from each other. The spores and tetraspores are equally capable, like buds, of reproducing their species; but the spores are believed to be in some cases fertilized by spindle-shaped particles, and consequently are considered to be the true fruit. Antheridia, or sacs containing these particles, have been discovered in various genera of Rhodosperms. Although, as a rule, the red Algæ have two modes of vegetative reproduction, yet there are various species in which tetraspores only have hitherto been met with. [Illustration: Fig. 24. Vertical sections of conceptacles:—_a_, Gracilaria armata; _b_, Grinnelia americana; _c_, Corallina officinalis, the membrane of which, more highly magnified, is shown at _d_.] A large proportion of the higher Rhodosperms is distinguished from those possessing the preceding mode of fructification by the internal structure of their reproductive nuclei. In some of these Algæ the nuclei are divided into two equal chambers by a fibro-cellular substance to which the spores are attached; in others, pear-shaped spores radiate from a fibro-cellular substance at the base of the nucleus. There are, moreover, Algæ which have nuclei containing conical spores whose broad bases radiate from the centre, and other arrangements occur.